1. Field of the Invention
The present invention relates to an ink-jet recording method and an ink-jet recording apparatus.
2. Description of the Related Art
Many ink-jet recording apparatuses (hereinafter, also referred to as “printers”) have been sold on the market for monochrome or full-color printing. These printers employ an ink-jet recording method in which a liquid or fused ink is discharged from nozzles, slits, a porous film or the like to record images on a recording medium such as a paper sheet, a cloth, or a film. Such printers are advantageous due to their compactness, inexpensiveness, and silence. Among them, printers utilizing a so-called piezo ink-jet recording method in which a piezoelectric element is used, or a so-called thermal ink-jet recording method in which droplets are formed with thermal energy to record images have many advantages such as high-speed printing, and high resolution.
The above-described ink-jet recording methods generally utilize inks which contain water-soluble dyes (hereafter, also referred to as “dye inks”). These inks possess excellent long-term storage stability, however, they are problematic in that they do not have sufficient resistance to water and light. In contrast, inks containing pigments (hereafter, also referred to as “pigment inks”) possess excellent resistance to water and light, and further, they produce image quality with high density and no blotting or blurring. For this reason, pigment inks have been very promising and many proposals have been made and put into practice utilizing this technology.
Japanese Patent Application Laid-Open (JP-A) No. 56-147871 proposes a pigment ink made from an aqueous medium containing at least a pigment, a polymer dispersant, and a nonionic surfactant. U.S. Pat. Nos. 5,085,698 and 5,221,334 propose the utilization of an AB or BAB block copolymer as a pigment dispersant. U.S. Pat. No. 5,172,133 teaches a pigment ink which uses a specified pigment, a water-soluble resin, and a solvent.
Due to the fact that pigments are not water soluble, a diepersant is used to disperse them throughout the ink. In contrast to this, research is being conducted on developing self-dispersing pigments that can easily disperse through ink via hydrophilic processing. Such pigment inks are described in various patent documents, such as JP-A Nos. 8-3498 and 8-31944, where pigment inks using a carbon black pigment are taught. In these applications, the carbon black has a surface-active hydrogen content of 1.5 mmol/g obtained through oxidation treatment. Another pigment ink is taught in Japanese National Publication No. 10-510862, where the ink uses a hydrophilic pigment prepared by introducing a hydrophilic group into the surfaces of carbon black particles via a connecting group, which is comprised of an aromatic group and analkyl group. Further, in JP-A No. 10-110129, an ink using a pigment processed with a sulfonating agent is described.
When utilizing pigment inks, however, the image quality varies to a large degree depending on the type of paper used. Depending on the paper, there are cases where the optical density of a printed image is low. This is due to the fact that the optical density of pigment inks is obtained by the trapping of the ink, mainly in the surface vicinity of the paper. This is in contrast to dye inks, where the color material actually dyes the paper fibers. It is thought that in pigment inks, the balance between the penetration and coagulation of the pigment in the paper surface greatly varies depending on the type of paper used.
It is possible to improve the technology and reduce the optical density variance caused by varying paper types. For example, the diameters of the particles dispersed in the ink can be increased; the number of the particles, made from a water-insoluble substance such as a pigment having particle diameters of 0.5 μm or more, can be increased; or the coagulability of the ink's colorant can be enhanced.
When ink contains water-insoluble substance having dispersed particles with increased diameters, where the number of particles with diameters of 0.5 μm or more is increased, and the coagulation of the pigment is also increased, problems can occur. For example, if a printer is left unused for a long period of time, the ink nozzles tend to clog when printing is restarted.
Therefore, there is a need for an ink-jet recording method in which an ink does not easily cause a nozzle to clog when printing is reinitiated after a long period of non-use. Moreover, these is a need for an ink-jet recording apparatus implementing this method.